1. Field of the Invention
The invention pertains to olefin polymerization. Specifically, it relates to the use of a select organosilicon compound as part of a catalyst system for alpha olefin polymerization. In a particular practice, the invention leads to increased catalyst activity without sacrifice of stereoregularity in polymerization of alpha olefins of three carbons or more.
2. Description of the Prior Art
While numerous catalysts are known to effectuate polymerization of alpha olefins, those employed commercially are invariably constituted of one or more transition metals. These in turn are almost uniformly utilized in conjunction with various other compounds to enhance activity. For example, industrial polymerization of alpha olefins using transition metal catalysts typically employ an aluminum alkyl compound as a cocatalyst to improve yield.
Other externally added compounds are sometimes used to promote select polymer characteristics. For example, in propylene polymerization, not only is yield a consideration, so is stereoregularity. This is because unlike ethylene, polymerization of propylene results in a polymer whose backbone has methyl groups pendent thereto. Three possible spacial configurations can result: the methyl groups can all be on the same side of the backbone plane (isotactic polypropylene), or they can they can regularly alternate (syndiotactic polypropylene), or they can be randomly placed (atactic polypropylene).
These conformations have important economic implications because they affect the thermal and mechanical properties of the resultant polymer. Thus isotactic polypropylene is highly crystalline, dense, strong and hard; it is accordingly one of the more high volume and commercially significant thermoplastics. Atactic polypropylene, on the other hand, is low in crystallinity, and is typically sticky, soft and weak; it unsurprisingly has few commercial applications.
Because a stereoregular structure is preferred, much effort has been expended to foster this in the polymerization reaction. Conventional Ziegler-Natta catalysts benefit from a tendency to favor the production of the isotactic form in the first instance. Nonetheless, various techniques have been explored to further the stereoregular proclivity of such catalysts. One of the most common involves the use of an external electron donor.
External donors are generally thought to improve isotacticity by, among other things, selectively deactivating aspecific sites or altering same to isospecific sites. In the ordinary course, the external donors most often used are Lewis bases. External donors of initial industrial interest have included aromatic esters, such as ethyl benzoate, methyl toluate, and ethyl anisate; as well as various amines, such as tetramethylpiperdine. One of the more recent classes of compounds to capture commercial attention in this regard are the silanes. Typically, these are compounds of the formula R.sub.n Si(OR').sub.4-n, where R and R' are hydrocarbyl and n is 1, 2 or 3.
Conventional wisdom dictates that to afford maximum activity and stereoregularity, silanes of the above type must have the bulkiest R groups practicable concurrent with the smallest possible alkoxy (OR') groups. Accumulated knowledge also prescribes that the R groups should be limited to those containing only hydrogen and carbon so as to avoid any untoward electronic affects. Thus use of R groups such as phenyl, cyclopentyl, cyclohexyl and isobutyl in tandem with methoxy groups (OR') are customary.
Despite the utility of these compounds, some are difficult to obtain commercially or synthetically whereas others have proven to be deleterious to catalyst performance or to the overall polymerization reaction. Hence there is an ongoing need to identify materials that will enhance activity, and do so without adversely affecting stereoregularity in situations where this is a concern.